Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells.
A thin-film photovoltaic or optoelectronic device is ordinarily manufactured by depositing material layers onto a substrate. A thin-film photovoltaic device ordinarily comprises a substrate coated by a layer stack comprising a conductive layer stack, at least one absorber layer, optionally at least one buffer layer, and at least one transparent conductive layer stack.
The present invention is concerned with photovoltaic devices comprising an absorber layer generally based on an ABC chalcogenide material, such as an ABC2 chalcopyrite material, wherein A represents elements in group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(In,Ga)Se2 semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indium-selenide or copper-gallium-selenide, in the form of quaternary, pentanary, or multinary materials such as compounds of copper-(indium, gallium)-(selenium, sulfur), copper-(indium, aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium, gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium, sulfur).
The photovoltaic absorber layer of thin-film ABC or ABC2 photovoltaic devices can be manufactured using a variety of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spraying, sintering, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple evaporation sources. Historically derived from alkali material diffusion using soda lime glass substrates, the effect of adding alkali metals to enhance the efficiency of thin-film ABC2 photovoltaic devices has been described in much prior art (Rudmann, D. (2004) Effects of sodium on growth and properties of Cu(In,Ga)Se2 thin films and solar cells, Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved 2012 Sep. 17 from <URL: http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf>).
Much prior art in the field of thin-film ABC2 photovoltaic devices mentions the benefits of adding alkali metals to increase photovoltaic conversion efficiency and, of the group of alkali metals comprising elements Li, Na, K, Rb, Cs, best results have been reported when diffusing sodium from precursor layers (see for example Contreras et al. (1997) On the Role of Na and Modifications to Cu(In,Ga)Se2 Absorber Materials Using Thin-MF (M=Na, K, Cs) Precursor Layers, NREL/CP-520-22945), or also EP0787354 by Bodegaard et al., or as well US20080023336 by Basol). More recent prior art provides data regarding diffusion of sodium and potassium from an enamelled substrate while also mentioning that potassium is known to dope CIGS in a similar way as sodium and hinders the interdiffusion of CIGS elements during growth of the absorber layer (Wuerz et al. (2011) CIGS thin-film solar cells and modules on enamelled steel substrates, Solar Energy Materials & Solar Cells 100 (2012) 132-137). Most detailed work has usually focused on adding or supplying sodium at various stages of the thin-film device's manufacturing process. Although often listed among other alkali metals, the beneficial effects of specifically adding, in a controlled manner, very substantial amounts of potassium, possibly in combination with some amount of sodium, has been insufficiently explored in prior art (see for example page 66 of Rudmann, D. (2004)). Section 4.2.2 of Rudmann, D. (2004) underlines a less pronounced beneficial effect of potassium in comparison to that of sodium. For reference, the highest photovoltaic conversion efficiency achieved in prior art for a photovoltaic cell on a polyimide substrate, i.e. on a potassium-nondiffusing substrate, with an ABC2 absorber layer where sodium is added via physical vapor deposition of NaF, is about 18.7%, as reported in Chirila et al. (2011) Nature Materials 10, 857-861.
Prior art has so far not specifically disclosed how adding, in a controlled manner, substantial amounts of potassium to layers of thin-film ABC2 photovoltaic devices can, especially in combination with sodium, enable the production of a class of photovoltaic devices with superior photovoltaic conversion efficiency. Prior art does not disclose how much potassium should be comprised within devices resulting from a controlled addition. In the field of manufacturing of flexible photovoltaic devices, there is a strong need for know-how regarding the controlled addition of alkali metals since some lightweight flexible substrates such as polyimide do not comprise the alkali metals known to passively diffuse out of rigid substrates such as soda-lime glass or enamelled substrates.
Furthermore, most prior art has assumed that sodium and potassium have similar effects on absorber layer and the optoelectronic device, such as doping, passivation of grain boundaries and defects, elemental interdiffusion, the resulting compositional gradients, and observed optoelectronic characteristics such as enhanced open circuit voltage and fill factor. This assumption has hindered inventiveness with respect to controlled addition of alkali metal combinations. This invention exploits previously unexplored properties of adding specific combinations of potassium and at least one other alkali metal, such as sodium, to a thin-film optoelectronic device, and especially to its absorber layer. The invention discloses independent control of separate alkali metals during adding to layers of the optoelectronic device. Besides aforementioned effects such as doping, passivation of grain boundaries and defects, elemental interdiffusion, and observed optoelectronic characteristics such as enhanced open circuit voltage and fill factor, the invention's adding of alkali metals enables manufacturing of a thinner optimal buffer layer. This thinner optimal buffer layer results in reduced optical losses, thereby contributing to increase the device's photovoltaic conversion efficiency. This invention not only specifies a method to add potassium, but also the amount of potassium that should remain in the resulting thin-film device and, in the case sodium is also added, the ratio of potassium to sodium.
Finally, manufacturing of embodiments of photovoltaic devices on polyimide substrates according to the method, and at what a person skilled in the art would consider low and unfavorable temperatures, has resulted in a photovoltaic conversion efficiency that is greater, at filing date, than the highest ever certified using similar absorber layer technology but manufactured at the more favorable high temperature processes allowable by glass substrates. This suggests that the invention contributes a step that may overcome the need for high temperature processes or even benefit them too.